200
M. H. Köhler et al.
can make a certain material a good candidate to be used in desalination membranes.
High structural strength when subjected to high pressures is certainly one of them.
Therefore, as important as the ability to retain ions is the membrane’s ability to remain
stable under strong external pressures. Unfortunately, an experimental work by Sun
et al. [15] showed that nanochannels created in lamellar thin films of WS 2 with the
addition of metal hydroxide nanostrands can crack under high pressure. Although
first-principles calculations revealed Young’s modulus Y = 200 GPa for 2D WS 2 ,
comparable to stainless steel, finite-element-based mechanical simulations confirmed
the collapse of cylindrical channels at a critical pressure of ~ 1.6 GPa. All of these
structural fractures could pose a significant problem regarding the membrane’s desalination capacity. Remarkably, the formation of nanocracks within the WS 2 membrane
was found to increase water permeance as a consequence of increased porosity. In
other words, the crack produces new fluidic nanochannels that further results in water
flux four times higher than that of the as-prepared WS 2 membrane, without rejection
performance degradation. According to Sun’s group, these membranes have separation performances 2 orders of magnitude higher than that of commercial membranes
with similar rejection rates.
3 MoS 2 and WS 2 Syntheses and New Fabrication Methods
Usually, 2D materials are the basic building blocks of bulk materials when they
are stacked together layer by layer. The possibility to obtain a single layer of carbon
(graphene) from bulk graphite has led to exciting new physics and material properties,
for instance: atomically thin electrical and thermal conductors, high transparency to
visible light, and high carrier mobility [28]. After the first works on graphene isolation by micromechanical exfoliation, several other methods to obtain graphene were
investigated, such as molecular beam epitaxy [29], CVD on metal surfaces [30], and
graphene films on silicon carbide (SiC) single crystals formed by silicon evaporation
[31], among others. Yet, none of these growth methods completely fulfills every basic
requirement for applications where suitable size and quality are mandatory, such as
in high-performance electronic devices. Still, some of these growth methods such as
CVD and silicon evaporation from SiC crystals can be used in applications with less
restricted requirements, namely: anti-corrosion coatings and paints [32], nanoporous
membranes for desalination [33], and pesticide biosensing [34]. Moreover, the electronic band structure of graphene has a linear dispersion near the K point, where the
bottom of the conduction band touches the top of the valence band, making it a zerogap semiconductor. This provides a great challenge to produce electronic devices for
many applications where high “on and off” electrical current ratios are required.
In the past decade, many other 2D materials have been isolated and studied, such as
hexagonal boron nitride (h-BN), silicene, phosphorene, and TMDs. TMDs are a class
of materials composed of a transition metal from groups IV to VI (such as Ti, Nb, W,
Mo, etc.) and a chalcogen (S, Se, or Te). Different from graphene, these 2D materials
are composed of three layers of atoms, where a metal plane is bonded to a chalcogen
M. H. Köhler et al.
can make a certain material a good candidate to be used in desalination membranes.
High structural strength when subjected to high pressures is certainly one of them.
Therefore, as important as the ability to retain ions is the membrane’s ability to remain
stable under strong external pressures. Unfortunately, an experimental work by Sun
et al. [15] showed that nanochannels created in lamellar thin films of WS 2 with the
addition of metal hydroxide nanostrands can crack under high pressure. Although
first-principles calculations revealed Young’s modulus Y = 200 GPa for 2D WS 2 ,
comparable to stainless steel, finite-element-based mechanical simulations confirmed
the collapse of cylindrical channels at a critical pressure of ~ 1.6 GPa. All of these
structural fractures could pose a significant problem regarding the membrane’s desalination capacity. Remarkably, the formation of nanocracks within the WS 2 membrane
was found to increase water permeance as a consequence of increased porosity. In
other words, the crack produces new fluidic nanochannels that further results in water
flux four times higher than that of the as-prepared WS 2 membrane, without rejection
performance degradation. According to Sun’s group, these membranes have separation performances 2 orders of magnitude higher than that of commercial membranes
with similar rejection rates.
3 MoS 2 and WS 2 Syntheses and New Fabrication Methods
Usually, 2D materials are the basic building blocks of bulk materials when they
are stacked together layer by layer. The possibility to obtain a single layer of carbon
(graphene) from bulk graphite has led to exciting new physics and material properties,
for instance: atomically thin electrical and thermal conductors, high transparency to
visible light, and high carrier mobility [28]. After the first works on graphene isolation by micromechanical exfoliation, several other methods to obtain graphene were
investigated, such as molecular beam epitaxy [29], CVD on metal surfaces [30], and
graphene films on silicon carbide (SiC) single crystals formed by silicon evaporation
[31], among others. Yet, none of these growth methods completely fulfills every basic
requirement for applications where suitable size and quality are mandatory, such as
in high-performance electronic devices. Still, some of these growth methods such as
CVD and silicon evaporation from SiC crystals can be used in applications with less
restricted requirements, namely: anti-corrosion coatings and paints [32], nanoporous
membranes for desalination [33], and pesticide biosensing [34]. Moreover, the electronic band structure of graphene has a linear dispersion near the K point, where the
bottom of the conduction band touches the top of the valence band, making it a zerogap semiconductor. This provides a great challenge to produce electronic devices for
many applications where high “on and off” electrical current ratios are required.
In the past decade, many other 2D materials have been isolated and studied, such as
hexagonal boron nitride (h-BN), silicene, phosphorene, and TMDs. TMDs are a class
of materials composed of a transition metal from groups IV to VI (such as Ti, Nb, W,
Mo, etc.) and a chalcogen (S, Se, or Te). Different from graphene, these 2D materials
are composed of three layers of atoms, where a metal plane is bonded to a chalcogen